1. Field of the Invention
The present invention relates to a charge control circuit capable of controlling the charging of a secondary battery, and a rechargeable power source apparatus using the circuit.
2. Description of the Related Art
As a conventional charge control circuit, there is known a circuit such as the circuit block diagram shown in FIG. 2. In such a circuit, in order to control a charger 208 through a voltage of a secondary battery 201, two systems of power sources, that is, the secondary battery 201 and the charger 208 exist in this charge control circuit. A charge-discharge control circuit 202 for monitoring the voltage of the secondary battery 201 and for controlling charge and discharge is operated by the secondary battery 201 as the power source, while a charge switch control circuit 207 for driving a charge control switch FET-B211 is operated by the charger 208 or the secondary battery 201 through the FET as the power source. At this time, in the case where the voltage of the secondary battery 201 becomes the minimum operation voltage of the charge-discharge control circuit 202 or lower due to abnormal deterioration, breakage, short circuit, or the like of the secondary battery, the charge-discharge control circuit 202 with the power source of the secondary battery 201 does not operate normally, and the output to the charge switch control circuit 207 becomes unstable. Hence, it becomes impossible to operate the charge control switch FET-B normally. As a result, there is a possibility that abnormal charging is performed on the secondary battery 201. Thus, according to the charge switch control circuit 207 shown in FIG. 3, a lowering of voltage of the secondary battery 201 is detected through the threshold voltage of a MOS transistor 304, and charging to the abnormal battery is prohibited.
In the case where the battery voltage of the secondary battery 201 is monitored through the threshold voltage of the MOS transistor 304, if the voltage of the secondary battery 201 is not higher than the threshold voltage of the MOS transistor 304, the charge control switch FET-B is turned off to prohibit charging. If the voltage of the secondary battery 201 is not lower than the minimum operation voltage of the charge-discharge control circuit 202, the charge control switch FET-B is turned on and charging is made possible. However, when the voltage of the secondary battery is in a region between the threshold voltage of the MOS transistor 304 and the minimum operation voltage of the charge control circuit 202, it is impossible to control whether the charge control switch should be tuned on or off. Moreover, even if the voltage of the secondary battery 201 is not higher than the threshold voltage of the MOS transistor 304, the charge control switch FET-B may be turned on by a leak current of the MOS transistor 304. As a result, there is a defect that charge prohibition can not be completely assured unless the voltage of the secondary battery 201 becomes sufficiently lower than the threshold voltage of the MOS transistor and becomes almost 0 V.
The present invention has been made to solve the problem of the conventional technique as described above. Therefore, an object of the present invention is to provide a charge control circuit having high reliability and safety in which a MOS inverter circuit is used so that when the voltage of a battery is not higher than a threshold voltage of a MOS transistor, a charge control switch is not turned on by a leak current of the MOS transistor, and when a charger is connected at the time when the voltage of the secondary battery is not higher than a threshold voltage of the Mos inverter circuit, charging current is made not to flow through the battery to cause such a state that charging can not be made.
In order to achieve the above object, in a charge control circuit according to the present invention, a circuit structure is designed such that in the case where a voltage of a secondary battery becomes a threshold voltage of a MOS inverter circuit or lower and a charger is connected at that time, a switch circuit is turned OFF and a charging current is made not to flow, so that charging is not conducted on a battery in an abnormal state.
In the charge control circuit structured as described above, if the charger is connected at the time of abnormal battery connection when the battery voltage is not higher than the threshold voltage of the MOS inverter circuit, the charging current is cut off and charging of the secondary battery becomes impossible. The circuit then operates to prevent breakage of the secondary battery, raising the reliability of the whole apparatus and improving safety.